1. Field of the Invention
This invention relates to process systems in which downstream gas or vapor pressure fluctuations can adversely affect an upstream process, e.g., reaction, deposition or coating systems involving discharged gas streams which are in fluid flow communication with upstream components or unit operations of the process system. More particularly, the invention relates to apparatus and method for stabilizing the pressure fluctuations attendant the use of such systems, in applications in which the gaseous effluent streams are generated by upstream processes which are adversely affected, e.g., susceptible to upset, by downstream pressure fluctuations.
2. Description of the Related Art
In the field of treatment of gaseous effluents in the manufacturing of semiconductor materials, devices, and products, storage and memory articles, and the use of photolithography for oligonucleotide characterization, a wide variety of effluent gases are produced in the process facility.
A large number of these effluent gases contain volatile organic compounds (VOCs), such as alkanols, organics, photoresists, and breakdown products of photoresists and other reagents, and a wide variety of other gases which are desirably removed from the waste gas streams produced in the process facility, before the waste gas is vented from the process facility to the atmosphere. For example, VOCs present in the exhaust streams of semiconductor manufacturing tools include isopropyl alcohol, photoprocessing vapors, and spin-on glass (SOG) solvents.
The normal options for the treatment of VOC-containing gas streams include combustion and catalytic oxidation, which in either case may further include preliminary concentration of the gas stream VOC components to be removed by the treatment process.
Combustion processes are old and well-established in the art for the treatment of VOC-containing waste gas streams, however, catalytic oxidation processes are in increasingly widespread use as a result of their high efficiency and cost-effectiveness. By catalytically converting the VOCs oxidatively to the combustion products carbon dioxide and water, the catalytic oxidation process affords an effective solution to the problem of VOC-containing waste stream treatment.
In catalytic oxidation treatment, the VOC-containing stream is heated to an elevated temperature appropriate to the oxidation reaction, and the resulting heated effluent gas stream then is contacted with an oxidation catalyst to oxidize the VOC components of the effluent gas stream to yield carbon dioxide and water.
The treated gas stream, at elevated temperature, then may be passed through a heat exchanger in heat exchange relationship with the feed effluent gas stream to the treatment system to recover the sensible heat of the treated gas stream and concurrently preheat the feed effluent gas stream, particularly when it is desired to carry out autothermal catalytic oxidation.
Catalytic oxidation may advantageously be conducted in a multibed system for continuous treatment of the VOC-containing effluent gas stream. In such multibed systems, a bed of oxidation catalyst is provided in each of multiple vessels, with a bed of thermal ballast (heat sink) material upstream from the catalyst bed. At any given time, at least one bed is on-stream, through which the VOC-containing effluent gas stream is cyclically sequentially and repetitively flowed for contacting with the oxidation catalyst therein. Concurrently, the non-active vessel(s) of the system, i.e., those not removing VOCs, are being utilized for heat recovery, by passage of the effluent gas stream, reduced in VOC content and at high temperature from the catalytic oxidation in the active vessel, from the active vessel outlet discharge end, to the non-active vessel, so that the sensible heat of the effluent gas stream is given up to the thermal ballast material in the non-active vessel.
Advantageously, the hot gas stream from the active vessel is flowed countercurrently with respect to the flow of the VOC-containing gas stream through the active vessel. In one useful apparatus arrangement, the outlet ends of respective vessels in the treatment system may be manifolded or otherwise disposed in flow communication with one another, so that the discharged (catalytically oxidized) stream from the active vessel is passed to the outlet discharge end of the non-active vessel for flow through the non-active vessel from its outlet discharge end to its inlet end, and final discharge from such inlet end to a discharge means for the catalytic oxidation system.
The multiple vessels of the VOC-abatement catalytic oxidation system thus may be interconnected by valved manifolds at their inlet/outlet ends, and arranged, e.g., with cycle timer means, so that one of the vessels is actively on-stream, in VOC-abating operation, with the VOC-containing stream being flowed therethrough for treatment, while other vessel(s) are undergoing heat recovery of the sensible heat from the VOC-reduced gas stream (regeneration), with cyclic switching of the vessels, so that the fully regenerated vessel resumes active operation, and the active vessel then undergoes regeneration, to accommodate continuous treatment of the VOC-containing gas stream.
In a simplest of such manifolded vessels arrangement, two vessels are arranged so that one is on-stream, while one is off-stream (undergoing regeneration), and with cyclic reversing operation of each of the vessels between the on-stream and off-stream states, involving reversal of the flow direction of the gas stream through the each of the vessels. In a linked two vessel arrangement, the VOC-containing gas stream is flowed through a first vessel from the inlet to the outlet end thereof, undergoing catalytic oxidation in such vessel. The VOC-depleted gas stream discharged from the outlet end of the first bed then is flowed in the reverse direction, from the outlet end to the inlet end, of a second vessel, for regeneration thereof. After the vessels have been operating in such mode for a first predetermined time, the valves in the manifold at the inlet end of the vessels are switched, to divert the VOC-containing gas stream from the first vessel to the second vessel.
After a second predetermined time period during which the VOC-containing gas stream is flowed to the inlet of the second vessel for passage therethrough from the inlet end to the outlet end thereof, and the VOC-depleted gas stream discharged at the outlet end of the second vessel is reverse-flowed through the first vessel, from the outlet end to the inlet end thereof, for regeneration of the first vessel, the vessels are again switched, by appropriate opening and closure of the valves in the inlet manifold joining the inlet ends of the respective first and second vessels in selective flow communication with the source of the VOC-containing gas stream.
During the switching of the respective beds in the above-described VOC abatement catalytic oxidation system, the contemporaneous opening and closure of the valve in the feed manifold of the multibed unit causes a pressure pulsation or surge, as the valves in the manifold open and shut, to divert the VOC-containing stream from one vessel to the other.
Such pressure surge, incident to the switching of the manifold valves in the multibed system of manifolded vessels, is particularly disadvantageous in process systems where the VOC-containing gas stream is generated in a pressure-sensitive or flow-sensitive operation, such as the aforementioned spin-on-glass processes, or other coating unit operations or steps, such as are widely employed in the semiconductor industry. There thus is a switchover perturbation in the catalytic oxidation system, during which there is no flow through any of the vessels in the multivessel system, and during which a pressure wave travels sonically upstream in the effluent treatment system, to the source of the VOC-containing vapors.
This back-surge then can severely adversely affect the upstream process. In instances where the VOC-containing gas derives from a coating or deposition process step, the pressure wave can radically change the pressure conditions in the deposition or coating chamber, and the thickness of the laid-down material will correspondingly radically vary from point to point on the substrate. In consequence, the substrate structure being processed may be rendered wholly unsuitable for its intended purpose.
For example, the layers being formed on a substrate may be grossly over-thick or under-thick, or may be highly uneven within the desired uniform coating region of the base structure. The process as a result of the pressure perturbation may therefore yield defective or unusable product. This thickness problem involves depth of focus considerations in the semiconductor industry, where the depth of focus of the steppers which are exposing the wafers at small line geometries may be no greater than the thickness of the resist itself, e.g., on the order of one micron and below, as the line size geometries employed. In such applications, thickness variations in the resist will cause certain portions of the resist to be out of focus.
Such thickness variation is an enormous problem in wafer processes, particularly when photoresist is being applied onto what may already be a tortuous or otherwise significantly nonuniform topography. Thus, any factors which cause the resist layer thickness to vary appreciably may severely impact wafer quality, such as coater bowl exhaust flow rate.
Currently, it is reported that resist is being commercially applied to six-inch wafers within a tolerance of .+-.3 Angstroms, and efforts are being made to reduce thickness variations even further, below this level.
In such systems, any significant pressure variations which are propagated from the downstream exhaust treatment system to the upstream resist application process can greatly disrupt the uniformity of the applied resist coating. The same is true of other film formation, coating, and deposition operations which are "coupled" in the gas phase with a downstream exhaust gas treatment system.
The foregoing discussion has been directed to exhaust gas treatment catalytic oxidation systems such as are widely employed for the treatment of process gases from semiconductor manufacturing operations. In addition, various regenerative oxidation systems are employed for treatment of effluent gas streams deriving from process facilities such as semiconductor manufacturing operations. Such regenerative oxidation systems are susceptible to the same problems and deficiencies as discussed hereinabove in reference to catalytic oxidation systems.
Further, there exist a wide variety of other process systems which involve flow circuits in which pressure waves, surges or spikes are detrimental and result from the switching of process flows in the system. Such flow circuits may involve a wide range of unit operations, including phase separation, chemical reaction, solubilization, physical mixing, gas-liquid contacting, distillation, etc., in which the flow is periodically reversed, diverted or otherwise switched from one flow path or flow direction to another. The present invention is directed to a solution to the pressure variation problems incident to such system flow changes.
Relative to the present invention hereinafter more fully disclosed, relevant art in the general field of the present invention is discussed below.
U.S. Pat. No. 5,361,800 issued Nov. 8, 1994 to James H. Ewing discloses a liquid delivery and vaporization system including a positive displacement pump assembly for delivering a continuous volume flow at a constant rate to a vaporizer assembly for flash vaporizing the liquid. This effort at solving the problems attendant the occurrence of pressure oscillations in a process system utilizes a mechanical actuated butterfly valve. This control system reads exhaust pressure from a CVD tool and reacts to the pressure sensing by incrementally opening or closing the valve. However, typical high pressure oscillations have frequencies in the range of from about 0.05 to 0.20 second. The valve control system described in the Ewing patent has difficulty responding to oscillations in this frequency range.
A mechanical progressive valve is commercially available from Progressive Technologies, Inc. which includes an exhaust controller, but such valve assembly is highly specific to the source of the effluent gas (i.e., "tool-specific" in relation to process tools used in semiconductor manufacturing facilities), and is relatively expensive.
U.S. Pat. No. 5,118,286 issued Jun. 2, 1992 to Michael C. Sarin describes a method and apparatus for operating a semiconductor wafer processing furnace in which multiple wafers are positioned in a reactor tube, and reaction gas is passed through the tube between the wafers. An inlet of an exhaust gas tube is located downstream from the wafers in the reactor tube. Spent reaction gas flows through the exhaust gas tube and an exhaust valve connected to the exhaust gas tube, and into an exhaust gas scrubber system. The total gas pressure in the reaction tube near the inlet of the exhaust tube is measured relative to ambient atmospheric pressure by a differential manometer and the flow of spent gases in the exhaust tube is controlled by comparing the pressure-indicating signal to a preset signal indicative of a preselected desired constant pressure to produce an error signal. The exhaust valve is automatically controlled as to its open character by the magnitude of the error signal. The differential manometer and control circuitry described in this reference are disclosed to control the measured pressure in the reactor tube with an accuracy of approximately 0.01 torr above ambient atmospheric pressure.
Other art of interest includes: U.S. Pat. No. 5,211,729 issued May 18, 1993 to Robert C. Sherman (baffle/settling chamber for removal of solid particulates from exhaust of semiconductor deposition equipment while reducing pressure fluctuation in the exhaust); U.S. Pat. No. 4,834,020 issued May 30, 1989 to Lawrence D. Bartholomew, et al. (atmospheric pressure chemical vapor deposition system with metering device connected to exhaust allowing continuous removal of reactant products, with wire scraper orifice cleaning arrangement); U.S. Pat. No. 4,993,358 issued Feb. 19, 1991 to Imad Mahawili (CVD reactor with independently adjustable multiple gas inlet orifices and exhaust ports); U.S. Pat. No. 5,113,789 issued May 19, 1992 to George D. Kamian (self-cleaning flow control orifice mounted in exhaust line for CVD apparatus); U.S. Pat. No. 5,136,975 issued Aug. 11, 1992 to Lawrence D. Bartholomew, et al. (injector with plates including hole arrays, defining a cascaded hole matrix arrangement for provision of uniform flow); and U.S. Pat. No. 5,450,873 issued Sep. 19, 1995 to David W. Palmer, et al. (regulator including a path, through which fluid passes, and a movably mounted piston having a frontal face, which is exposed to fluid in the path at a constriction point, and a distal face, which is exposed to a reference pressure, with the piston is mounted so that the weight of the piston exerts a force on the piston in a direction that tends to widen the path at the constriction point, and with a force being exerted on the piston, preferably by a spring under compression, in a direction that tends to narrow the path at the constriction point).
It therefore would be a significant advance in the art, and is therefore an object of the present invention, to provide a means and method for modulating and suppressing the effects of pressure perturbations incident to flow changes in a flow circuit, e.g., such as result from the periodic cyclic switching of valves or other flow control means (e.g., fluidic gates or routers, caps, closures, thermostatic acutators, etc.) in the flow circuit.
It is an object of the invention to provide an improved means and method for damping pressure perturbations incident to switching of valves or other flow control means in a fluid flow circuit of an oxidation system, such as a catalytic or regenerative oxidation system for treatment of effluent gas from semiconductor manufacturing operations.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.